Server not found in Windows 7

Although there seems to be lots of searches on Google Bing etc all the answers appear to be trying to sell me something. Being a complete novice I at a loss as to what is the best route to take to clear up this problem. The error does not occur all the time and I cannot pinpoint it to any particular web site. The only way I have been able to get past the message is to click on the "try again" icon a few times!

I am using Windows 7 professional and connect to the internet with an ethernet connection through a router.
I really would appreciate some help here with this annoying problem.

I also get that quite often. I'm suspecting that temporarily The 'Net connection from your computer to the desired web site is not yet complete. It's a disconnect until the path from your computer's hard-drive to the web site host's hard-drive is complete. There is also server-caching galore going on, however, I do not have a clue as to how that connects and disconnects.

server not found

If you want help, then you have to give us more information....
Which browser, firewall, antimalware-program, etc. are you using ?

Sorry for lack of information but as I indicated I am a "newbie" and can only really try to answer questions as to what you may require to help me solve this problem

This is what I have so far
The PC is a Lenovo ThinkPad running Windows 7 Professional
I have a Dlink router which uses an ethernet cable to connect to the PC
(I have also tried a direct connection bypassing the router)
I have Microsoft Essentials - Super Antispyware program - Malewarebytes anti exploit.
and I am using Firefox version 43 browser - I also tried Windows explorer version 11.0.23 which gives the same error